Cyclosporin is a high molecular peptide compound consisting of 11 amino acids which achieves its potent immunosuppressive activity by inhibiting the growth and differentiation of T cells. There are many cyclosporins such as cyclosporin A, B, C, D, G, etc. depending on the structure of constituent amino acids, but cyclosporin A is preferably used in the clinical field since its pharmacological activity and clinical indication and effectiveness are well established in the art. Cyclosporin was found in Tolypocladium inflatum gams by Borel et al. in 1976, and developed as antibiotics at first. After it was found during the safety test that cyclosporin inhibits the growth of lymphocytes, cyclosporin has become the focus of the world's attention as the only immunosuppressant which can specifically affect only lymphocytes, and made the technical development in the organ transplantation possible.
Cyclosporin has a unique structure, which is a cyclic oligopeptide consisting of 11 amino acids. The seven amino acids of cyclosporin are N-methylated. The four remaining protonated nitrogen atoms can form intermolecular hydrogen bonds with carbonyl groups, which contribute substantially to the rigidity of the cyclosporin skeleton. Therefore, it has a remarkably hydrophobic property, and is relatively insoluble in water (for cyclosporin A, 0.04 mg/ml at 25.degree. C.). Due to such a low water-solubility of cyclosporin A, the bioavailability of cyclosporin A is known as be 30% or less. It was reported that the absorption of such an insoluble compound is greatly influenced by bile juice secretions, the amount of fat in food, etc. In case of cyclosporin A, it has been reported that differences in absorption between individuals are as great as about 5.about.50%. When cyclosporin is administered for a long time, it shows renal toxicity, liver toxicity, etc. The renal side effects comprise a reduction of glomerular filtration, an increase in proximal renal tubular reabsorption and chronic progressive deterioration of nephron.
Cyclosporin has a large dosage unit and a narrow therapeutic index as well as the said properties, and the condition of patient to be treated with cyclosporin is generally unstable. Therefore, it is very difficult to establish an optimum drug dosage regimen for survival of transplanted patient through maintenance of efficient and constant blood concentration that can prohibit side effects and rejection. Numerous studies have been conducted to overcome the said properties and to develop an improved pharmaceutical formulation. Such studies have been mainly concentrated on the means that are used to solubilize cyclosporin. Typical examples include not only mixed solvent systems consisting of vegetable oil and surfactant, but also microspheres, the formations of powdery composition using adsorption, inclusion complexes, solid dispersions, and other numerous formulations. An oral preparation containing cyclosporin as the primary active ingredient has been commercialized in a solution form and a soft capsule formulation. The preparation uses a mixed vehicle consisting of vegetable oil, surfactant and solvent to solubilize cyclosporin, but did not overcome the cyclosporin properties, that is, low bioavailability of cyclosporin and large individual difference thereof. Therefore, the preparation has many problems in the clinical use.
Microemulsions were first reported by J. H. Schulman in 1943 and have, since then, been mainly studied for application of cosmetics and as carrier of insoluble drugs. A microemulsion comprises two or more immiscible liquid materials together with surfactant like emulsion, but is a thermadynamically stable and optically transparent unlike emulsion. And the microemulsion has very low surface tension and small particle size of less than 100 nm, which together result in high absorption and permeation properties of drug delivered by microemulsion. The microemulsion is, especially, very useful in solubilization and absorption improvement of insoluble drugs. However, since formation of a microemulsion requires a lot of surfactant, severe mucosal irritation is caused by the microemulsion preparation, and the volume of dosage becomes large. In medical field, microemulsions have thus been utilized only for preparations that are applied to the skin such as preparations for hair, detergents, etc.
In case of cyclosporin as the relative insoluble drug, U.S. Pat. No. 4,388,307 discloses an oral liquid preparation, which includes oil, surfactant, and ethanol as the hydrophilic solvent. This preparation is a microemulsion preconcentrate and, therefore, must be diluted with water before it is administered orally. That makes the patient compliance bad, and makes the control of the exact dosage difficult. Since it is also uncomfortable to carry, it is impossible to actually administer to a patient who must undergo cyclosporin therapy for the remaining period of his life.
In order to remove the said disadvantages of liquid preparations, a microemulsion preconcentrate formulated in the form of soft capsule has been developed. The microemulsion preconcentrate, until now, has comprised drug, hydrophilic solvent, surfactant and oil with appropriate mixing ratio, and could form the microemulsion spontaneously through dissolving in the outer phase such as water and intestinal fluid. However, the hydrophilic solvent used as essential component of microemulsion preconcentrate permeates the gelatin shell of the capsule not only to volatilize, but also to soften the gelatin shell. Those induced the instability of the soft capsules.
In case of the cyclosporin soft capsule containing ethanol as a hydrophilic component, this capsule preparation must contain a large amount of ethanol to solubilize the cyclosporin. However, since ethanol permeates the gelatin shell of the capsule to volatilize even at normal temperature, the content of ethanol is reduced in the course of time. As a result, when the capsules are stored at low temperatures or at normal temperatures for long period, the crystallization of cyclosporin may be caused. The composition change results in a great variation of cyclosporin bioavailability and, therefore, it is impossible to obtain a desired therapeutic effect reliably and reproducibly. As an effort to prevent the volatilization of ethanol from the soft capsule preparations during storage, the soft capsules are enclosed in a special packaging material, such as aluminum blister packaging. However, the problem of great variation of cyclosporin bioavailability is still remained in spite of using the said special packaging since it is impossible to completely prevent the change of ethanol content in the course of time. Using the said special packaging might contribute to an increase of the manufacturing cost and medicinal fee.
To improve the said disadvantages of the use of ethanol as hydrophilic cosurfactant, methods using non-ethanol component as hydrophilic cosurfactant have also been proposed. U. S. Pat. No. 5,342,625 discloses the soft capsule preparation formulated with microemulsion concentrate which solves the said problems. This patent discloses a pharmaceutical composition in the form of microemulsion concentrate in which a pharmaceutically acceptable C.sub.1.about.5 alkyl or tetrahydrofurfuryl di- or partial- ether of a low molecular mono- or poly- oxy-alkanediol, for example, diethyleneglycol monoethyl ether (e.g. Transcutol), tetrahydrofurfurylalcohol polyethylene glycol ether (e.g. Glycofurol), 1,2-propylene glycol are used as a hydrophilic solvent, and ethanol is used as a hydrophilic co-solvent. However, all the hydrophilic solvents used in this patent are glycols having the alcoholic group (--OH) in their structure. Since such glycols containing --OH group are very hygroscopic, they absorb moisture present in atmosphere and further have a high permeability to gelatin shell. It is, therefore, very difficult to formulate a composition containing such a glycolic cosurfactant into a soft capsule preparation. In encapsulating and first drying steps at the time of preparing soft capsules, particularly, water that is present in the capsule shell is absorbed into the capsule content by the amount corresponding to 20% of the hygroscopic solvent to cause the change in constitutional ratio of the composition. And then, in drying step of those, water is distributed again into the gelatin shell and volatilized from inside of capsule to outside of that through the capsule shell. The composition materials of capsule content migrate also to capsule shell along with water. Therefore, the constitutional ratio of the composition according to this patent is greatly changed, and the change of the preparation appearance from the said phenomena makes the capsule production yield decreased.
The said hydrophilic solvents used in the formulation disclosed by the above U. S. patent have, furthermore, the softening effect for the gelatin shell of capsule, and induce the pharmaceutical problem that the appearance stability of gelatin capsule is greatly reduced. This problem becomes even more serious when a plasticizer for gelatin, for example, propylene glycol, glycerin, etc. is used as a hydrophilic solvent. The use of propylene glycol as the main solvent is, therefore, limited highly. Propylene glycol can be used generally in approximately 5% or less of capsule contents, and may be used in approximately 10% or less together with a hardening agent for gelatin shell at the most. The appearance stability of the gelatin shell is greatly reduced when propylene glycol is used in the amount of exceeding the said limits as the content of soft capsule.
Korean Patent Application No. 94-13945 discloses a cyclosporin-containing composition to be formulated in soft capsule (trade name: Neoplanta.RTM.) that uses dimethylisosorbide as the hydrophilic cosurfactant for mitigating such disadvantages. This patent describes that the soft capsule preparation using dimethylisosorbide does not show the appearance change of the soft capsule and the content change of the ingredients since dimethylisosorbide has substantially no permeability to the gelatin shell in comparison to the other hydrophilic cosurfactants used in the prior art.
Dimethylisosorbide, which is marketed under the trade name Arlasolve.RTM., i.e., 1,4:3,6-dianhydro-2,5-dimethyl-D-glucitol, is a solvent that has been generally used as a percutaneous absorption enhancer only for topical pharmaceutical ointments or cosmetics such as lotions.
As mentioned above, a conventional microemulsion requires more quantity of surfactant than a general emulsion does. Therefore, in case of a drug that is continuously administered to the patient during his whole life after organ transplantation, for example, cyclosporin, the toxicity of solvent and surfactant contained in the preparation of the microemulsion by long term administration must be also considered. In this connection, the LD.sub.50 value of dimethylisosorbide is 5.63 ml/kg (rat, per oral). The LD.sub.50 (rat, per oral) value of the organic solvent of which the toxicity has been relatively well known, is as follows: Acetonitrile, 3.8 g/kg; acetone, 10.7 ml/kg; benzene, 3.8 ml/kg; toluene, 7.53 g/kg; isopropanol, 5.8 g/kg; and butanol, 4.36 g/kg. It is expected that the long-term oral administration of the composition containing dimethylisosorbide may cause problems. Furthermore, the pharmaceutical problems were not improved highly by the use of dimethylisosorbide, since dimethylisosorbide still has the properties, the reactivity with gelatin shell of soft capsule and the volatility and shows the limit as hydrophilic solvent.
U.S. Pat. No. 5,583,105 discloses the oral multiple emulsion comprising cyclosporin in which ethanol and tocopheryl polyethylene glycol 1000 succinate, that is a surfactant, are used as essential vehicle, and in which oils or alkyl esters of polycarboxylic acid are used as lipophilic or amphiphilic solvent. This patent describes that alkyl esters of polycarboxylic acid can be used selectively instead of oil, and this patent, in particular, employed acetyl triethyl citrate for formulation of cyclosporin. However, ethanol, which is a hydrophilic and volatile solvent, was also used essentially to constitute the cyclosporin formulation. As mentioned above, the cyclosporin preparations using ethanol for the solubilization of cyclosporin show the problem of pharmaceutical stability, such as ethanol volatilization through the gelatin shell of capsule, during the storage. Tocopheryl polyethylene glycol 1000 succinate, which was used as a surfactant in this patent, is a product from esterification of tocopheryl acid succinate with polyethylene glycol, and liberates tocopherol after being absorbed into the body. This patent discloses that free tocopherol can reduce the renal toxicity. However, it has not been verified yet, and there could be a drug interaction between cyclosporin and free tocopherol, which may be absorbed in the body exceeding the usual dose depending on the amount of the composition administered, for a long therapy. Moreover, it has been known generally that the lipid-soluble vitamins, such as tocopherol, induce the adverse side effect by accumulation in the body.
As mentioned above, the microemulsion preconcentrates according to prior arts comprise hydrophilic solvent, oil and surfactant as essential and primary composition. In case of formulation of those microemulsion preconcentrates as soft capsule, it has been known generally that hydrophilic solvent reacts with gelatin shell of soft capsule thereby to soften the shell, and volatilizes through the gelatin shell, and that the said phenomena induce the serious problem of preparation stability. There was a try to improve the those disadvantage using dimethylisosorbide as new hydrophilic solvent, but this material has the limit as a primary vehicle because of toxicity thereof and still has the disadvantage of hydrophilic solvent.